In an FMCW radar system (frequency modulated continuous wave radar) it is already known to linearize the transmitter frequency, modulated in a ramp-shape, i.e. for the duration of a ramp the frequency change is constant per unit of time. The oscillator frequency is measured in the course of this and is supplied in a closed control loop in a control circuit to a controller. As a function of a comparison between command or a reference value and an actual value, the controller forms an adjusting or setting value which is supplied to the oscillator and which follows up the latter's frequency accordingly. It has been found that the tuning elements used, for example varactor diodes, only provide unsatisfactory results because of their non-linear characteristic curves. Added to this is that external influences, such as temperature or aging, change the characteristic curve of the tuning elements. In order to obtain satisfactory results for linearity in spite of this, very complicated measures, which generate relatively high expense, are required. It is furthermore disadvantageous that, as a rule, control circuits are susceptible to oscillations, so that there is no assurance of faultless operation of the radar system.
It is also known to measure the frequency lines of the oscillator as a function of the temperature prior to the actual use of the radar system and to form correcting values therefrom, which are stored. These values are then used for correcting the measured characteristic curve of the oscillator frequency. However, to execute this method it is necessary to use an additional temperature sensor with the aid of which it is possible to detect temperature-dependent correcting values. This method requires very large expenditures, since characteristic curves must be determined for each oscillator. But the problem of susceptibility to oscillations in the control loop cannot be affected with this method.